1. Field of the Invention
The present invention relates to an environmental isolation housing, and more particularly to an environmental isolation housing for a crystal used in a laser system.
2. Description of the Related Art
Crystals used in laser light applications may have some of their critical parameters altered by exposure to environmental factors. For example, Lithium Tri-Borate (LBO) is hydroscopic, and chemically re-acts with water vapor. Typically, when an LBO crystal is used in laser light applications, two opposing faces of the crystal will be polished optically flat, and anti-reflection coated, so that a laser beam will pass through the crystal with very small loss.
The anti-reflection coating can provide a barrier to the water vapor. However, the thermal expansion coefficients of LBO are very high and, at present, no available antireflection coating materials are similar, so very large mechanical stresses occur at the LBO-to-coating interface over the typical 25.degree. C. to 160.degree. C. operating range. At the edges of the coating, and at any tiny defect sites, water molecules may react with the LBO and destroy the bond between the LBO and the coating. This mechanism may damage or destroy the entire coating within a few weeks in a high humidity environment. Although moisture may also be absorbed through the unpolished sides of the crystal, it is absorbed very slowly and typically only causes damage at a greatly reduced rate compared to the damage cause by moisture absorption at the crystal face.
Other environmental factors in the atmosphere can also cause problems for crystals used in lasers. For example, dust particles and some vapors react with laser light in such a manner that they may move along the light beam toward an optical surfaces. Once they are attached to an optical surface they may scatter the light causing loss of power. Also, these contaminant can absorb the light and cause crystal heating that distorts the laser beam wavefront or possibly damages the surface of the crystal via thermally induced stresses.
Typically, optical elements used with laser light have been protected from dust and vapor contamination by sealing the optical elements in a very clean assembly or by purging the assembly with a high-purity gas, usually air or nitrogen.
Complete sealing, using clean room techniques, is satisfactory if no field service or change of the optics is anticipated. A purge gas system allows the optics or other components to be changed in the field. However, it does not protect the optics during shipment or at times when the purge gas system is not operating unless the gas pumping and purification system is a sealed part of the assembly.
Additionally, alignment of the laser optical components is usually very critical, often requiring mechanical alignment and stability within 0.0005.degree.. Complete sealing has the complication that altitude or other atmospheric pressure changes produce substantial forces on the mechanics of the optical system which can misalign them. Very stiff structures or symmetrical designs can be used to counter the pressure changes, but sometimes more deliberate pressure equalizing mechanisms are needed.
Precise alignment of the optical components of a laser is also complicated by the sealing system used. The seals must have controlled flexibility or be adjustable using only small forces.
There is a need for crystal isolation methods which consider all the chemical, optical, and precise mechanical requirements of lasers outlined above.